Amplified nucleoli can be isolated from Xenopus oocytes completely free of any DNA other than ribosomal DNA. These nucleoli are transcriptionally active in vitro and apparently retain most of the nucleolus-specific proteins. We will use these miniature chromosomes to immunize mice and prepare hybrid cell lines producing monoclonal antibodies against nucleolar proteins. With a library of such antibodies we will ask questions such as: 1. What is the composition of active versus inactive ribosomal gene chromatin? 2. Where do various antigens map on the ribosomal DNA? 3. Can we find antibodies that will block specific steps in RNA transcription? In general the monoclonal antibody technology should give us the ability to develop highly specific and sensitive probes against proteins that control nucleolar function. These probes permit a variety of experiments aimed at understanding the metabolism of this important organelle.